Feature & Refactor: read and write Hexx(R) in CSR format

[maki49]

Background

Currently, EXX-NSCF suffers from cereal's bug in reading Hexx(R) (related issues: #2235 #3323), which forces the users to turn to PyATB.

What's changed

• refactor (unify) the CSR writers
• read and write Hexx(R) in CSR format to bypass cereal
• add a parameter "reduce" to CSR writers: if false, each process will write its data respectively to its file.

[PeizeLin]

All HexxR have been reduced to only one process. Will this lead to memory crashes in large-scale computing?

[maki49]

All HexxR have been reduced to only one process. Will this lead to memory crashes in large-scale computing?

exx_lri.Hexxs is already a global matrix (with the same value on all the processors) no need for extra reduction.
What I did is simply that: if DRANK==0, convert to CSR and write; else, do nothing.

[PeizeLin]

All HexxR have been reduced to only one process. Will this lead to memory crashes in large-scale computing?

exx_lri.Hexxs is already a global matrix (with the same value on all the processors) no need for extra reduction. What I did is simply that: if DRANK==0, convert to CSR and write; else, do nothing.

For each process, only part of Hexx is stored in memory.
Although the same Hexx[I][J] may appear repeatedly in multiple processes, it does not mean that each process contains all Hexx.
For large systems, the memory requirement to store the global Hamiltonian matrices is unacceptable.

[maki49]

For each process, only part of Hexx is stored in memory. Although the same Hexx[I][J] may appear repeatedly in multiple processes, it does not mean that each process contains all Hexx. For large systems, the memory requirement to store the global Hamiltonian matrices is unacceptable.

So there's a waste of memory in Hexx to be improved. Maybe I can have a try in another PR.
This PR is to circumvent cereal's bug, as long as to "convert to CSR and write" in every processor can achieve this goal, right?

[PeizeLin]

For each process, only part of Hexx is stored in memory. Although the same Hexx[I][J] may appear repeatedly in multiple processes, it does not mean that each process contains all Hexx. For large systems, the memory requirement to store the global Hamiltonian matrices is unacceptable.

So there's a waste of memory in EXX Hexx to be improved. Maybe I can have a try in another PR. This PR is to circumvent cereal's bug, as long as to "convert to CSR and write" in every processor can achieve this goal, right?

right